Genetics
Since human females have two X chromosomes and males have one X and one Y (usually; there are cases where it is not so clear), diseases or other things can be restricted to one sex.One example is colorblindness; about 5.8% of males, but less than 1% of females, are colorblind in some way or another—although it should be noted that males get their X chromosome from their mother, so whether one's father is colorblind has no influence on whether one is colorblind himself.
Cats actually have sex-linked coat patterns that are probably the closest thing to a real-life example, although coat patterns can occur in multiple breeds: almost all calico and tortoiseshell cats are female.
This can be even more true with some insect species that have fewer chromosomes to worry about. At least one species of ants actually has the genetic material of the two sexes completely separate.
